The present invention relates to a method of controlling the quantity of light in a selected range in applications using a semiconductor laser for writing data, as in a laser printer, a laser facsimile transmitter/receiver, a digital copying machine, and the like.
In a laser printer which employs a semiconductor laser, it has been customary to vary the printing (image) density of a hard copy as follows:
The intensity of light emitted from the semiconductor laser is detected, and the detected light intensity (voltage Vx) is compared with a preset reference level (reference voltage Vref). The level of a drive current applied to the semiconductor laser is controlled, based on the result of the comparison, by a feedback control loop provided by an automatic power control (APC) circuit so that the light emission intensity will correspond to the reference value (see Japanese Laid-Open Patent Publication No. 62-116071).
More specifically, a controlled power level (initial level) achieved by the APC and a preset emission intensity level are added, and the drive current for the semiconductor laser is determined based on the sum through a D/A converter. The preset emission intensity level is of a digital value of "100000", for example, and converted by the D/A coverter into analog value. The value of the drive current for the semiconductor laser may be expressed as .DELTA.i/DATA=100000.
Since the D/A converter has a constant gain, .DELTA.i is proportional to the digital value at a constant gradient as shown in FIG. 3.
One characteristic of a semiconductor laser is its differential efficiency .eta. defining the gradients of straight-line curves X, Y (FIG. 4(I)) which represent the relationship between emission intensities P (on the vertical axis) and drive currents I (on the horizontal axis). The differential efficiency .eta. indicates the gradient of the light emission output with respect to the drive current. This characteristic tends to vary from semiconductor laser to semiconductor laser even if the lasers are of the same type, and also tends to vary with time even for the same semiconductor laser.
Where the above process of driving the semiconductor laser based on the sum of the initial level and the preset emission intensity level is employed for setting an emission intensity level, emission intensity levels may vary from laser to laser.
As described above, with the conventional light control process, the initial level and the preset intensity level are added for driving the semiconductor laser. Therefore, the light emission intensity can vary from laser to laser dependent on setting conditions, and this can make it difficult to print hard copy with good density.
It is an object of the present invention to provide a multilevel light quantity control method which will eliminate the aforesaid problems and can establish a light emission intensity level highly accurately regardless of different characteristics of semiconductor lasers and time-dependent changes thereof, in variably controlling the quantity of output light emitted from a semiconductor laser within a range (between first and second reference levels). Another object is to otherwise impose print quality.